This invention generally relates to solid oxide fuel cells. More particularly, the present invention relates to an improved solid oxide fuel cell design that allows multiple cells to be interconnected in both series and parallel, while allowing a single cell in a stack of cells to be removed and replaced.
A fuel cell is basically a galvanic conversion device that electrochemically reacts a fuel with an oxidant within catalytic confines to generate a direct current. A fuel cell typically includes a cathode material that defines a passageway for the oxidant and an anode material that defines a passageway for the fuel. An electrolyte is sandwiched between and separates the cathode and anode materials. An individual electrochemical cell usually generates a relatively small voltage. Thus, to achieve higher voltages that are useful, the individual electrochemical cells are connected together in series to form a stack. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
The fuel and oxidant fluids are usually gases and are continuously passed through separate cell passageways. Electrochemical conversion occurs at or near the three-phase boundary of the gas, the electrodes (cathode and anode) and electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.
Specifically, in a solid oxide fuel cell (SOFC), the fuel reacts with oxide ions on the anode to produce electrons and water, the latter of which is removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxide ions that diffuse through the electrolyte to the anode. The electrons flow from the anode through an external circuit and then to the cathode, with the circuit being closed internally by the transport of oxide ions through the electrolyte.
In a SOFC, the electrolyte is in a solid form. Typically, the electrolyte is made of a nonmetallic ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons which ensures that the electrons must pass through the external circuit to do useful work. As such, the electrolyte provides a voltage buildup on opposite sides of the electrolyte, while isolating the fuel and oxidant gases from one another. The anode and cathode are generally porous, with the anode oftentimes being made of nickel/YSZ cermet and the cathode oftentimes being made of doped lanthanum manganite. In the solid oxide fuel cell, hydrogen or a hydrocarbon is commonly used as the fuel, while oxygen or air is used as the oxidant.
As mentioned above, the voltage output of a single fuel cell is far to low for many applications. Thus, it frequently becomes necessary to connect multiple fuel cells in series. Additionally, the power demands of many systems require that fuel cells frequently be connected in electrically parallel circuits, thereby providing a greater total current. The physical stacking of multiple fuel cells in series, parallel or series/parallel configuration, however, must incorporate gas-tight connections to allow for a safe and efficient flow of reaction gases. Typically, a group of individual fuel cells are welded, soldered or otherwise bonded together into a single unitary stack, thereby preventing the improper mixing of the reaction gasses, such as in U.S. Pat. No. 5,861,221. Accordingly, if one cell must be removed and replaced, such as for testing, the remaining cells are destroyed in the process. This leads to significant losses in time and money.
For example, defects can occur during firing of the cell materials, which negatively affect the performance of the fuel cell. Where adjacent cells are fused or bonded together into a single unitary stack, a single cell that is defectively formed cannot be removed and interchanged with a non-defective cell. At best, the performance of the fuel cell stack becomes impaired. At worst, the entire stack must be discarded due to the failure of a single cell.
An examination of the prior art reveals that the various constructions adopted in stacking multiple fuel cells in series and/or parallel fall short of easy replacement of individual fuel cells. In U.S. Pat. No. 3,776,777, an array of fuel cells forms a hollow cylinder. A series electrical connection is established by joining two half-cylinder portions. Electrodes are configured in bands across the inner and outer surfaces of the half-cylinder. An oxidizing agent is exposed to the outside of the cylinder, and a hydrogen fuel flows through the hollow center of the cylinder. The half-cylindrical portions are cemented together either by welding, or applying a ceramic or vitreous sintered layer. Thus, if a single fuel cell becomes defective, the entire cylinder must be discarded. Additionally, the geometric shape limits the use of such a structure where space is a limitation. It is not xe2x80x9cstackablexe2x80x9d in the geometric sense that a stack of essentially flat, thin fuel cells would form.
A unitary modular fuel cell stack in U.S. Pat. No. 5,741,605 can be fabricated outside a power generator and subsequently mounted therein. Each stack includes a plurality of tubular solid oxide fuel cells xe2x80x9cbundledxe2x80x9d into a semi-rigid unit. When a stack malfunctions, the stack may be removed without shutting down the entire power generation plant. No discussion, however, is offered on the design of a cell that can be individually removed from a stack.
U.S. Pat. No. 5,882,809 claims to provide individual cells in a stack that can be individually interchanged without impairing the stack, but only in the absence of high power density. The cells are individually sintered and mechanically held together. The mechanical means can include external clamps, springs or weights, although employing the same is not specifically described. Nevertheless, it can be surmised that such external components can only add to complexity in design and limitations in use. Further, there appears to be no discussion of how the individual cells are manifolded to fuel and oxidant gases while maintaining the ability to be individually removed.
As can be seen, there is a need for a unitary solid oxide fuel cell that can be removably installed in a fuel cell stack. Also needed is a unitary cell that can be replaced from a fuel cell stack without impairing the performance of the overall stack, such as by damaging adjoining cells. Another need is for a unitary cell that can be in a series and/or parallel electrical configuration with other fuel cells. An individual fuel cell and fuel cell stack design that is simple and cost effective is also needed. Yet a further need is for an individual cell and fuel cell stack design that does not require precise alignment from one cell to the next so as to prevent overall stack misalignment. In other words, a fuel stack design is needed that does not require precise flatness from one cell to the next.
The present invention provides a unitized fuel cell comprising a first electrically conductive interconnect operatively connected to an anode of the fuel cell, with the first interconnect having a first substantially planar portion and a first skirt portion; a second electrically conductive interconnect operatively connected to a cathode of the fuel cell, with the second interconnect having a second substantially planar portion and a second skirt portion, with the second skirt portion being juxtaposed to the first skirt portion; a gas inlet fixed to at least one of the first and second skirt portions; and a gas outlet fixed to at least one of the first and second skirt portions, whereby the gas inlet and outlet can be releasably attached to a gas manifold.
In another aspect of the present invention, a solid oxide fuel cell stack has a plurality of unitized solid oxide fuel cells, at least one unitized cell comprising (a) a first electrically conductive interconnect operatively connected to an anode of the one fuel cell, with the first conductive interconnect having a first substantially planar portion and a first skirt portion; (b) a second electrically conductive interconnect operatively connected to a cathode of the one fuel cell, with the second conductive interconnect having a second substantially planar portion and a second skirt portion, and the second skirt portion being juxtaposed to said first skirt portion; (c) a first salient formed by a portion of at least one of the first and second skirt portions, with first salient being disposed at a first edge of the one fuel cell; and (d) a second salient formed by a portion of at least one of the first and second skirt portions, with the second salient being disposed at a second edge of the one fuel cell; and a plurality of gas interconnects that releasably connect the first and second salients to a plurality of gas manifolds, whereby the unitized cells are disposed in the fuel cell stack in the absence of being fixed to one another such that a single unitized cell can be removed and replaced in the fuel cell stack.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.